1. Field of the Invention
The present invention relates to a method for cleaning a silicon substrate used for production of VLSIs.
2. Description of the Related Art
In the production of VLSIs, a liquid mixture of ammonia, hydrogen peroxide and water (hereinafter, referred to as an "ammonia/hydrogen peroxide cleaning liquid" or simply as a "cleaning liquid") is generally used for removing particles of silicon and silicon oxide adhering to a silicon substrate. When ammonia in the ammonia/hydrogen peroxide cleaning liquid etches the particles, the silicon substrate is also etched. Accordingly, non-soluble particles formed of, for example, resins such as TeflonR adhering to the silicon substrate are removed.
Recently, for the purpose of achieving higher integration of VLSIs, reduction in the size of the chips and in the thickness of the gate oxide film of the transistors is an important matter of research. In order to realize such reduction, the parameters in all the steps of the VLSIs production should be precisely controlled. For example, it is necessary that cleaning performed before the formation of a thin oxide film on the silicon substrate should be done in such a manner that the thickness of the film formed by natural oxidization after the cleaning will be accurately controlled. In the cleaning performed after the formation of a gate oxide film having a thickness of as small as several nanometers, slight fluctuations in the etching rate of the oxide film of approximately several tens of nanometers per minute presents a problem. Under these circumstances, cleaning parameters such as the thickness of the film formed by natural oxidization and the etching rate of the gate oxide film should be precisely controlled in addition to the ability of removing the particles.
Conventionally, a silicon substrate is cleaned by use of an ammonia/hydrogen peroxide cleaning liquid in the following manner.
A reagent of 30 wt. % aqueous solution of ammonia, a reagent of 29 wt. % aqueous solution of hydrogen peroxide, and water are mixed in a volume ratio of 1:1:5 in a bath to obtain a liquid mixture containing ammonia at a concentration of 4.3 wt. % and hydrogen peroxide at a concentration of 4.1 wt. %. The resulting liquid mixture is heated to a prescribed temperature. Thereafter, the silicon substrate is immersed in the resulting liquid mixture for a prescribed period of time, is rinsed with pure water, and is dried. Regarding such a method, refer to, for example, W. Kern et al., RCA Review, page 207 (1970).
Briefly referring to FIG. 6, characteristics of such a conventional method will be described. FIG. 6 is a graph illustrating the concentrations of ammonia and hydrogen peroxide in the conventional ammonia/hydrogen peroxide cleaning liquid changing in accordance with time at a temperature of 60.degree. C. Since ammonia is volatile in the ammonia/hydrogen peroxide cleaning liquid, the concentration of ammonia reduces in accordance with time, thereby changing the composition of the cleaning liquid. As is described in, for example, Japanese Laid-Open Patent Publication Nos. 2-159029 and 5-259141, the etching rate of a silicon oxide film depends on the concentration of ammonia in the conventional ammonia/hydrogen peroxide cleaning liquid. Accordingly, in the conventional ammonia/hydrogen peroxide cleaning liquid, the etching rate of the silicon oxide film changes due to the reduction in the concentration over time.
As is described above, the particles of silicon or silicon oxide adhering to the silicon substrate are removed by ammonia. Accordingly, the ability of removing the particles is higher as the concentration of ammonia is higher.
In the conventional ammonia/hydrogen peroxide cleaning liquid, the concentration of ammonia is initially sufficiently high to remove the particles but decreases in accordance with time. Since it is conventionally considered that a lower concentration of ammonia reduces the ability of removing the particles, the ammonia/hydrogen peroxide cleaning liquid is abolished after a certain period of time, and another bath of cleaning liquid is newly produced. Such a method increases the costs for the chemicals. Further, the cleaning parameters cannot be maintained constantly as the concentration of ammonia will always continue to reduce after the new bath of cleaning liquid is produced.
In an attempt to solve the above-described problem, Japanese Laid-Open Patent Publication No. 2-159029 describes a method for maintaining the concentration of ammonia in the ammonia/hydrogen peroxide cleaning liquid in the vicinity of 4.3 wt. % by adding the reagent of aqueous solution of ammonia in a prescribed amount at a prescribed time interval, thus keeping the cleaning characteristics at a constant level. However, when the concentration of ammonia is in the vicinity of 4.3 wt. %, the cleaning characteristics change drastically in correspondence with the concentration of ammonia. Accordingly, the cleaning characteristics significantly change by even slight fluctuations in the amount of the aqueous solution of ammonia added. For this reason, it is very difficult to control the concentration of ammonia by this method.
Japanese Laid-Open Patent Publication No. 2-159029 also describes a method in which the concentration of at least one of the components of the cleaning liquid is measured, and the necessary component is added in a necessary amount when necessary. It is described in this publication that the measurement of the concentration should be performed after the added component is mixed with the cleaning liquid uniformly. The measurement performed immediately after the addition does not present an accurate concentration since it takes a certain length of time for the added component to be mixed with the cleaning liquid uniformly. By the time the added component is uniformly mixed, the concentration of the component is already reduced by volatilization. Further, until the added component is uniformly mixed, the concentration of the component in the cleaning liquid bath is not uniform, thereby causing non-uniform etching on one same surface of the silicon substrate and also among a plurality of silicon substrates. The problem of non-uniform etching becomes more serious as the silicon substrate is increased in size.